Mr. Gorka came out swinging again on Thursday, after Mr. Trump’s contentious news conference in which he excoriated the media. Asked by Evan Davis of the BBC to assess Mr. Trump’s appearance, Mr. Gorka repeatedly declared the president’s performance “fabulous.”

“It’s only weird to journalists like yourself, who are biased,” he said.

Who is he? Here’s what we know:

Naturalized American citizen

Mr. Gorka is an American citizen who was born in Britain to Hungarian parents. He earned a Ph.D. in political science from Corvinus University of Budapest and has made his living as a national security expert with a focus on Islamist extremism.

Breitbart veteran and friend of Bannon

Mr. Gorka, 46, is a former editor for the far-right media outlet Breitbart News and a friend of Stephen Bannon, the former Breitbart chairman who is now a powerful assistant to Mr. Trump.

Until recently, Mr. Gorka was not well known to Washington policy makers. But his increasing visibility has brought headlines, some less welcome than others.

He has appeared in a number of television and radio interviews as a representative of the Trump administration and a member of a White House team called the Strategic Initiatives Group. The Daily Beast called it a think tank within the White House that was set up by Mr. Bannon and the president’s son-in-law and senior adviser, Jared Kushner.

‘Who is he reporting to?’

The group’s formation raised red flags, said Julianne Smith, a former deputy national security adviser to Vice President Joseph R. Biden Jr. and the director of strategy and statecraft at the Center for a New American Security.

The National Security Council has traditionally played a decisive role in foreign policy decisions, she said. “Now we have the Strategic Initiatives Group and the National Security Council both working on issues of national security and strategy. So my question on Sebastian, ultimately, is: Who is he reporting to? Is he reporting to the National Security Council? Or is this a direct line to Bannon?”

Mr. Gorka said he could not comment for this article without clearance from the White House. But after multiple calls and emails since Tuesday, neither he nor the White House has answered questions about his position or the role of the initiatives group.

This month, Mr. Gorka told CNN that the group would bring private industry expertise to bear on a range of issues, including cybersecurity, veterans affairs and the modernization of government technological systems.

“That is very different from what the National Security Council is doing every day under the sterling leadership of General Flynn,” he added, referring to Michael T. Flynn, then the national security adviser, who would resign 12 days later.

Ms. Smith said she was not personally familiar with Mr. Gorka, though she had crossed paths with him during the early 2000s.

“I don’t know what to believe about this guy, but given his experience at Breitbart and what I had heard years ago, it does appear to me that he takes some rather extreme views,” she said.

Claims of Nazi sympathies

Mr. Gorka dismissed such criticisms during a Wednesday interview with Sean Hannity of Fox News and brushed aside some of the more controversial reports buzzing around him.

“I think we’re doing our job very well — don’t you, Sean? — if this is the best they can do,” he said.

He referred to reports, like one from Fusion, that said he had exaggerated his role as an expert witness in the trial of Dzhokhar Tsarnaev, who was convicted of helping to carry out the Boston bombing of 2013. Mr. Gorka told Mr. Hannity he still had invoices for the expert report he submitted for the case.

“I may not have taken the stand, but I was an expert on the Boston bombing trial,” he said.

As for the speculations about Nazi sympathies, they go back to one of Mr. Trump’s inauguration balls, when Mr. Gorka — appearing in photos and a video interview with Mr. Hannity — wore a medal that could be interpreted as a nod to Miklos Horthy, a Hungarian leader who entered into a strained alliance with Nazi Germany in the early years of World War II.

In a video posted by Breitbart on Tuesday, Mr. Gorka said the medal belonged to his father, who he said had suffered “under both the Nazis and the Communists” in Hungary, having lived through World War II and survived torture under the Soviet-imposed government that later came to power.

Misdemeanor weapons charge

Mr. Gorka was charged with a misdemeanor after he took a handgun into Reagan National Airport in Virginia last year.

In a statement quoted by Breitbart, he said that on his way to the airport, he had “grabbed the wrong bag, one I had just used” at a gun range. He added that he was allowed to board his flight, and that the weapon was confiscated.

Though his appointment to the White House occurred only recently, Mr. Gorka has been in touch with Mr. Trump at least since 2015. He wrote last year that he had submitted some policy papers to the presidential candidate and had met with him twice to discuss national security issues.

It is difficult to predict the impact Mr. Gorka could have on foreign policy in the White House, Ms. Smith said, especially since he is apparently working under the shadowy aegis of Mr. Bannon.

“We’ve got a situation where both outsiders and certain members of the government are confused about who is serving as the lead on foreign policy decisions,” she said. “And because we are not clear about Steve Bannon’s role, it raises a lot of concern because of his personal views, and also his lack of experience.”